1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to fabrics, such as forming fabrics, for use with a papermaking machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process, which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Among others, the properties of surface smoothness, absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam.
The present invention relates specifically to the forming fabrics used in the forming section. Forming fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to form and convey the paper product being manufactured to the press section or next papermaking operation.
The upper surface of the forming fabric, to which the cellulosic fibrous web is applied, should be as smooth as possible in order to assure the formation of a smooth, unmarked sheet. Quality requirements for forming require a high level of uniformity to prevent objectionable drainage marks.
Of equal importance, however, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Contemporary forming fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric that may be woven from monofilament yarns and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, metal or other material suitable for this purpose and known by those of ordinary skill in the paper machine clothing arts.
Those skilled in the art will appreciate that most forming fabrics are created by flat weaving, and having a weave pattern which repeats in both the warp or machine direction (MD) and the well or cross-machine direction (CD).
The design of forming fabrics typically involves a compromise between the desired fiber support and fabric stability. A fine fabric having small diameter yarns and a high number of yarns in both the MD and CD directions may provide the desired paper surface and fiber support properties, but such a design may lack the desired stability and wear resistance resulting in a shorter useful fabric life. By contrast, a coarse fabric having larger diameter yarns and fewer of them may provide stability and wear resistance for long service life at the expense of fiber support and the potential for marking. To minimize the design tradeoff and optimize both support and stability, multi-layer fabrics were developed. For example, in double and triple layer fabrics, the forming side is designed for fiber support while the wear side is designed for strength, stability, drainage, and wear resistance.
Many fabrics today, especially triple layer fabrics, comprise two separate fabrics (two complete weave patterns) which are held together by either MD or CD binder yarns as part of the weaving process. They therefore fall into the class of “laminated” fabrics.
However, a shortcoming of laminated fabrics is the relative slippage between the layers of the fabric. This slippage and relative fabric movement ultimately may lead to fabric delamination. Specifically, triple layer fabrics may have a top and bottom layer which may be held together by binder yarns. The top fabric layer may be a plain weave structure, which is designed for optimal paper sheet formation and fabric support. The bottom fabric layer may be designed for wear resistance and may be woven with long floats in which the weft monofilament travels under three or more warp monofilaments. These long floats may be used as an anti-abrasive wear surface. The binder yarn monofilament may be a weft monofilament that mechanically holds the top and bottom fabric layers together by traveling over at least one warp monofilament in the top fabric layer and under at least one warp monofilament in the bottom fabric layer. Under running conditions on the paper machine, the bottom and top fabric layers move relative to each other. This relative movement may lead to fatigue and wear of the binder monofilament due to repeated deflection back and forth within the structure. Eventually, the binder monofilament may fail and allow the top and bottom fabrics to separate (delaminate) from each other.
Further, the lamination of the fabric should not interfere with drainage of the structure such that the sheet of paper formed on the structure has an undesirable mark.
In addition, forming fabrics, especially thin fabrics, may also be prone to wrinkling or folding. Wrinkling or folding may be due to high “sleaziness” of fabric construction. High sleaziness means that the fabric does not have the necessary dimensional stability or CD stiffness to remain flat during operation.
In addition, thin fabrics with very fine MD yarns may have lower seam strength than fabrics with larger diameter yarns. Low seam strength can cause fabrics to prematurely tear during operation.
The present invention provides a fabric with meltable yarns. Such yarns have a melting point lower than the remaining yarns in the fabric. As a result, when the fabric is heated, meltable yarns melt without effecting the remaining yarns and may bond or fuse with yarns in contact therewith or in close proximity thereto. For example, meltable yarns may be formed from MXD6. A monofilament yarn formed from MXD6 is able to maintain its integrity even when the outer surface of the yarn melts. These bonded or meltable yarns may improve seam strength, eliminate edge curl, improve sheet formation, improve planarity, improve dimensional stability and reduce fabric sleaze in all types of fabric, including triple layer fabrics. Such triple layer fabrics may also have improved surface planarity and lower water carrying capacity.